1. Technical Field
This invention relates to coolable airfoils in general, and to trailing edge cooling hole configurations in coolable airfoils in particular.
2. Background Information
In modern axial gas turbine engines, turbine rotor blades and stator vanes require extensive cooling. A typical rotor blade or stator vane airfoil includes a serpentine arrangement of passages connected to a cooling air source, such as the compressor: Air bled from the compressor provides a favorable cooling medium because its pressure is higher and temperature lower than the core gas traveling through the turbine; the higher pressure forces the compressor air through the passages within the component and the lower temperature transfers heat away from the component. In conventional airfoils, the cooling air exits the airfoil via cooling holes disposed, for example, along both sides of the leading edge or disposed in the pressure-side wall along the trailing edge. Cooling is particularly critical along the trailing edge, where the airfoil narrows considerably. Most airfoil designs include a line of closely packed cooling holes in the exterior surface of the pressure-side wall, distributed along the entire span of the airfoil. A relatively small pressure drop across each of the closely packed holes encourages cooling air exiting the holes to form a boundary layer of cooling air (film cooling) aft of the holes that helps cool and protect the aerodynamically desirable narrow trailing edge.
Conventional pressure-side trailing edge cooling schemes represent a trade-off between cooling flow and mechanical durability. The narrow cross-section of the airfoil at the trailing edge makes it impractical to cool the trailing edge via an internal cavity adjacent the trailing edge. In place of the cavity it is known to extend diffused cooling holes through the pressure-side of the external wall upstream of the trailing edge. The size and number of the conventional cooling holes reflects the cooling air flow necessary to cool the trailing edge. The practical size and number of the holes are limited, however, by the thickness of the airfoil wall. If the diffused apertures are positioned too close to the trailing edge, the trailing edge becomes undesirably thin and consequently susceptible to mechanical fatigue. To avoid the fatigue, the diffused holes are moved forward. Film cooling effectiveness, however, is inversely related to the distance traveled by the film.
What is needed is an airfoil with trailing edge cooling apparatus with improved cooling and one with improved resistance to mechanical fatigue.